The rainbow trout (Oncorhynchus mykiss), a carnivorous fish species, displays a “glucose-intolerant” phenotype when fed a high carbohydrate diet. The importance of carbohydrate metabolism during embryogenesis and the timing of establishing this latter phenotype are currently unclear. In addition, the mechanisms underlying the poor ability of carnivorous fish to use dietary carbohydrates as a major energy substrate are poorly understood. It has recently been shown in trout that duplicated genes involved in glucose metabolism may be involved in establishing the glucose-intolerant phenotype. The aim of this study was therefore to provide new understanding of glucose metabolism during ontogenesis and nutritional transition, taking into consideration the complexity of the trout genome. Trout were sampled at several stages of development from fertilization to hatching, and alevins were then fed a non-, or a high carbohydrate diet during first-feeding. mRNA levels of all glucose metabolism-related genes increased in embryos during the setting up of the primitive liver. After the first meal, genes rapidly displayed expression patterns equivalent to those observed in the livers of juveniles. g6pcb2.a (one glucose-6-phosphatase encoding gene) was up-regulated in alevins fed a high carbohydrate diet, mimicking the expression pattern of gck genes. The g6pcb2.a gene may contribute to the non-inhibition of the last step of gluconeogenesis and thus to establishing the glucose-intolerant phenotype in trout fed a high carbohydrate diet as early as first-feeding. This information is crucial for nutritional programming investigations as it suggests that first feeding would be too late to programme glucose metabolism in the long term